When a restaurant roof gets neglected and grease is all over, it can be a real hassle.
This is a whole different animal than cleaning kitchen exhaust systems. You don't want to be too aggressive or you will damage the roof, if it isn't already damaged by the grease.
The first thing you want to do is get up any of the wet grease on the roof. Use a flimsy scraper but make sure you don't dig too deep and cut into the roof. If you want to be super careful, use plastic scrapers. If you feel like you are digging into the roof on any area, try using the scraper to pull the grease towards you instead so it doesn't dig at all.
The next step is to use peat moss or Spag from Driploc to get it into what is left over, then sweep that up off of the roof. You can get really aggressive with this stuf as it will not harm the roof. Rub it in really well and you will see that it gets most of the grease off.
You can leave the peat moss on or wash it off it is up to you. If you want to get up what the peat moss and scraping didnt, you can use the same chemicals you use for kitchen exhaust cleaning, they won't damage the roof.
When you do wash the roof with a pressure washer, use a 15° or 25° tip, you never want to use a 0° tip or a turbo nozzle on a roof, you may damage them.
If there is some really old hard stuff that just isn't coming off with the pressure washer, while the grease is still wet from the wash, apply a generous amount of Commercial Oxiclean to those areas. This will loosen up the hardened grease over time, and it should be much easier to remove next time you come to do the service.